Field of the Invention
The present invention relates to a method for setting a temperature rising profile to be used to increase the temperature of a sensor element in initiating the use of a gas sensor.
Description of the Background Art
Various gas sensors have been conventionally used to obtain the concentration of a desired gas component of a measurement gas. For example, a NOx sensor including a sensor element formed of an oxygen-ion conductive solid electrolyte such as zirconia (ZrO2) is well known as a device that measures the NOx concentration of a measurement gas such as a combustion gas (for example, see Japanese Patent Application Laid-Open No. 09-113484 (1997) and Japanese Patent No. 4344486).
Japanese Patent Application Laid-Open No. 09-113484 discloses a process of manufacturing a sensor element that forms a gas sensor for obtaining a NOx concentration through introduction of a measurement gas into a space (internal space) provided in the element. In particular, Japanese Patent Application Laid-Open No. 09-113484 discloses a so-called three-chambered sensor element having three internal spaces.
Japanese Patent No. 4344486 discloses the use of, for example, a curve obtained by approximating, to an exponential function (first order lag-time function), the change in the heater resistance value relative to the time passed for increasing the temperature of the sensor element.
In initiating the use of a gas sensor as disclosed in Japanese Patent Application Laid-Open No. 09-113484 and Japanese Patent No. 4344486, the temperature of a sensor element needs to be increased to a predetermined operating temperature by a heater included in the element such that the sensor element can perform measurement, and oxygen needs to be pumped out of an internal space, in which a measuring electrode is included, such that the oxygen concentration in the internal space becomes equal to or lower than a predetermined value at which substantially no oxygen is assumed to be present. The period of time, from a time at which a gas sensor is started to be energized for use to a time at which the gas sensor becomes available, is referred to as a light-off time.
The light-off time is preferably shorter and, to obtain a shorter light-off time, the sensor element is required to be heated to an operating temperature as quickly as possible. However, an excessively high temperature rising rate causes a crack in the sensor element due to a thermal stress acting on the sensor element.
In conventional cases, unfortunately, the following operations have been merely performed: temperature is increased at a constant temperature rising rate at which the occurrence of no crack has been empirically confirmed, or as disclosed in Japanese Patent No. 4344486, feedback control is used in which an increasing rate of heater resistance is reduced every time the measured heater resistance reaches a predetermined value. Such operations are not necessarily optimized from the viewpoints of the prevention or reduction of cracks and the reduction of a light-off time.